The present invention relates generally to a microwave oven having both microwave and electrical resistance heating capabilities and adapted for operation from a power source of limited current supplying capabilities and, more particularly, to such an oven which fully utilizes at all times the limited current supplying capabilities of the power source.
So called countertop microwave ovens have recently been introduced which are designed for operation from a 120 volt, 15 amp household branch circuit. To meet UL requirements, an appliance designed for operation from such a power source is limited to a maximum requirement of 13.5 amperes, which corresponds to approximately 1620 watts. This limited power source capability results in some particular problems.
Specifically, a typical countertop microwave oven microwave energy generating system requires a major portion of the available current. A typical microwave energy generating system comprises a magnetron which produces between 500 and 600 watts of output power at a frequency of 2450 MHz, as well as a suitable power supply for the magnetron. Such a system has an energy conversion efficiency in the order of 50%. In addition to the microwave energy generating system, a practical microwave oven includes a number of low power load devices such as lamps, blower motors, and control circuitry. Altogether, one particular commercially produced countertop microwave oven model draws approximately 11.2 RMS amperes from a 120 volt branch circuit when cooking with microwave energy alone.
In addition, due to the already limited power, supplementary electrical resistance heating elements, such as browning elements, should be operated so as to require substantially all of the available power.
As a result, for such an oven designed for operation from a 120 volt, 15 amp household branch circuit, as a practical matter the limited power available precludes the simultaneous energization of the microwave energy generating system and the supplementary electrical resistance heating units as their respective full rated power levels.
As one answer to the practical limitation on available power, countertop microwave ovens have resorted to a two-step cooking procedure whereby cooking by microwave energy is accomplished first, with the electrical resistance heating element de-energized. Next, the microwave energy source is de-energized and electrical resistance browning element is energized for the remainder of the cooking cycle.
As another answer to this practical limitation on available power, in accordance with the inventions disclosed and claimed in commonly-assigned copending application Ser. No. 911,555, filed May 31, 1978, by Raymond L. Dills; application Ser. No. 911,615, filed May 31, 1978, by Bohdan Hurko and Thomas R. Payne; and application Ser. No. 911,614, filed May 31, 1978, by Thomas R. Payne and Bohdan Hurko, effective microwave and electrical resistance heating is accomplished concurrently by various time ratio control systems which alternately energize the microwave energy generating system and the electrical resistance heating unit a plurality of times during each cooking operation. For a number of reasons described in more detail in those applications, this in effect time shares the available power and leads to superior cooking results.
With both the two-step cooking procedure previously employed and in the time sharing approaches described in the above-mentioned commonly-assigned copending applications, the current supplying capability of the power source is not utilized to the fullest extent possible. Since the current supplying capability is limited, it is desirable to utilize it to the fullest over an entire cooking operation so as to realize the shortest possible cooking time. More specifically, the electrical resistance heating units can quite easily be designed to draw substantially all the available current when energized. However, such close tailoring of the current requirements of the microwave energy generating system is generally not feasible from a practical point of view because the components of the microwave energy generating system are commercially available generally only in certain sizes. It is highly unlikely that the current requirements of a standard system would exactly coincide with the available current.
As a specific example, the exemplary microwave oven mentioned above requires approximately 11.2 RMS amperes when cooking with microwave energy. Since the microwave oven is intended for operation from a 120 volt line, fused to 15 amps, it could draw a maximum of 13.5 RMS amperes and still meet UL requirements. Thus under these conditions 2.3 RMS amperes are still available from the power source and, if not effectively utilized, a cooking operation results which is not as fast as it otherwise might be. However, during periods when the electrical resistance heating element is energized, the entire available 13.5 RMS amperes may be drawn for full utilization of the power source current-supplying capability.